Generally, a compression ring and an oil ring are attached as a set of a piston ring to a piston that performs reciprocating motion. The compression ring possesses a function to prevent a phenomenon that high-pressure combustion gas flows into a crank room side from a combustion room side (blow-by). On the other hand, the oil ring mainly possesses a function to suppress a phenomenon that excess lubricant on the inner wall of a cylinder enters into the combustion room side from the crank room side to be consumed (oil-up). Moreover, as a combination of a conventional standard piston ring, a combination of three rings having two compression rings, which include a top ring and a second ring, and one oil ring has been known.
Recently, along with the weight saving and high power output of an internal combustion engine, quality required to a piston ring has been becoming higher. Conventionally, as a method to improve durability of a piston ring for an internal combustion engine, wear resistance surface treatment such as nitriding treatment, ion-plating treatment, or hard chrome plating treatment has been performed on a sliding surface.
Among these surface treatments, because nitriding treatment shows especially excellent wear resistance properties, is has attracted attention as a surface treatment for a piston ring used under harsh operating conditions and has been widely used.
However, though a piston ring on which a nitriding treatment layer has been formed has good wear resistance properties, when mounted on an aluminum alloy piston, it has a tendency to increase wear of ring groove of the piston. Moreover, due to the wear of ring groove of the piston, as shown in FIGS. 1 (a) to (c), aluminum adhesion in which aluminum of lower surface of groove 11 of an aluminum alloy piston 10 adheres on a lower surface 3 of a piston ring 1 (FIG. 1(c)) occurs.
FIGS. 2(a) to (c) are charts showing the changes of surface roughness of an upper surface 2 and the lower surface 3 of a ring groove of a piston which were detected by a tracer type surface roughness test machine. As shown in FIG. 2, surface condition of the upper surface 2 and the lower surface 3 of the ring groove of the piston changes from normal condition (FIG. 2 (a)) to a roughened piston groove condition (FIG. 2 (b)) to aluminum adhesion condition (FIG. 2 (c)).
Note that in any of FIGS. 2 (a) to (c), horizontal axis show position of a piston and vertical axis show swell of the piston groove. (F) in the figures stands for front direction, while (AT) is for anti-thrust direction, (R) is for rear direction, and (T) is for thrust direction.
Moreover, FIGS. 3 (a) to (c) show aluminum adhesion mechanism. The lower surface 3 of the piston ring 1 and the lower surface of groove 11 of the aluminum alloy piston 10 comes into contact with each other via oxide films 8 (0.2 μm or below) formed on the surfaces respectively (FIG. 3 (a)), then stress of the oxide films 8 on contacting portions becomes high locally to break the oxide films 8, thus uniting Fe of the lower surface 3 of the piston ring 1 and Al of the lower surface of groove 11 of the aluminum alloy piston 10 (FIG. 3 (b)), and aluminum alloy 20 is deposited on the lower surface 3 of the piston ring 1. Note that enlarged view of the aluminum adhesion portion is shown in FIG. 4. In FIG. 4, reference numeral 20 is aluminum that is adhered and reference numeral 21 is joint part of Al and Fe.
As described above, when partial wear (also known as roughened piston groove) due to the adhesion phenomenon on a certain part of a groove of a piston occurs along with the up and down movement of a piston ring, sealing property of an internal combustion engine is deteriorated by blow-by of blow-by gas, leading to lowered output power. This phenomenon occurs within a short period of time in the lower side of a ring groove of a piston and influences durability of the internal combustion engine much. Therefore, many countermeasures against wear of piston groove have been conventionally proposed.
For example, to prevent direct contact between a piston and a piston ring, as a countermeasure against wear of piston groove, anodic oxide coating treatment, coating treatment, or matrix toughening treatment (inside of a piston) has been performed for a piston, while phosphate coating treatment or coating treatment has been performed on a piston ring. Or, as shown in FIGS. 5 (a) and (b), resin coating treatment 8 has been performed on the surface of a piston 10 and a piston ring 1 (for example, Defric (manufactured by Kawamura Research Laboratories).
Moreover, to solve the above-mentioned problem, a piston ring on which upper and lower surfaces or on lower surface, wear resistance treatment layer of nitrided layer or chrome plating layer is formed and upon that wear resistance treatment layer, polybenzimidazole resin film which includes solid lubricant has been developed (refer to Patent Reference 1).
In addition, a piston ring of which surface is coated by heat-resistant resin which includes solid lubricant has been developed by someone other than the applicant of the present application (for example, refer to Patent References 2 and 3).    Patent Reference 1: Japanese Published Unexamined Patent Application No. 07-063266    Patent Reference 2: Japanese Published Unexamined Patent Application No. 10-246149    Patent Reference 3: Japanese Published Unexamined Patent Application No. 11-246823